In recent years, in an optical communication field in which a demand for high speed and larger capacity is high, for realizing a processing with a higher speed, a use of an ultrashort optical pulse is progressed, of which pulse width is a couple of pico seconds or not longer than that. Such the ultrashort optical pulse is used as an information transmission media regarding the optical communication. And then it is able to realize an ultrafast transmission by transmitting an optical pulse to be added information thereinto with a short time interval of a couple of pico seconds or not longer than that.
Moreover, an investigation is actively progressed in recent years for searching a new potentiality in such the field, because it is able to use the ultrashort optical pulse as an optical pulse having not only a short width but also a high energy, for such as working upon a material, for observing a characteristic of a material in a high accuracy, or the like.
While, as a method for directly generating an ultrashort optical pulse, there are generally known a method that a signal is superimposed to an applied current of a semiconductor laser diode (LD) and then an intensity of an output light is directly modulated, and a method that an intensity of a continuous light wave (CW) output from an LD is modulated using an external modulator. However, there is an upper limit on a speed of response for the LD itself, for the semiconductor for generating an electric signal to be a modulation signal, or for the external modulator, and then it is quite hard to generate directly the ultrashort optical pulse having the width of not longer than a couple pico seconds. Thus, a new method for generating an ultrashort optical pulse is investigated, and then there become provided a variety of methods therefor as a result in recent years.
As a new method for generating an ultrashort optical pulse for example, there is provided a method for performing a reshaping to be a preferred waveform by compressing a width of an optical pulse to be a seed, with using a nonlinear effect (the Kerr effect) of an optical fiber and using a dispersion effect thereof. Here, performing a reshaping to be a preferred waveform by compressing a width of an optical pulse is hereinafter referred to as compressing an optical pulse, or to as a compression of an optical pulse. It is known that it is able to realize a compression of a pulse having a high quality using a dispersion decreasing fiber (DDF) of which a dispersion value of the fiber is decreased in a longitudinal direction thereof, or using a comb-like dispersion profiled fiber (CDPF) to be configured by connecting alternately a dispersion-shifted fiber (DSF) having a dispersion value as zero and a single mode fiber (SMF), by a method for compressing a pulse which is so-called as an adiabatic soliton compression, among heretofore known some methods for compressing a pulse.
Here, the pulse having the high quality means the pulse that a pulse shape thereof is fitted in a high accuracy to a sech function for example, and that a pedestal (which means an element not to be fitted to a shape of the function thereof at a foot part of the pulse as generally larger than a value of the function to be attenuated for a time delay), a nonlinearity of a frequency chirp (a time variation of an instantaneous frequency), or the like, are small. Here, sech x=2/(ex+e−x).
However, regarding the above mentioned DDF, it is not easy technically to realize a preferred characteristic thereof, and then it is at a state as difficult to put it to practical use, because of a low yield and a high manufacturing cost thereof.
On the contrary, regarding the CDPF, it is expected to put it to practical use, as there are advantages that it is easy to manufacture it, and that it is able to suppress the manufacturing cost thereof, because it is configured using two types of fibers for transmission.
Here, the compression of a pulse using the DDF or the CDPF is disclosed in nonpatent documents 1 and 2 respectively. Moreover, as a developed type of the CDPF, there is provided a comb-like profiled fiber (referred to as a CPF hereinafter) comprised of a configuration that a highly nonlinear dispersion-shifted fiber (HNL-DSF), which has a dispersion to be negligible and has a larger nonlinearity, or simply a highly nonlinear fiber (HNLF) is used in place of the DSF, and then the HNLF and the SMF are alternately connected to therebetween. Such the CPF gathers attention because it becomes able to realize a high compression efficiency with a shorter fiber length, and there is disclosed in a nonpatent document 3 regarding a detail thereof including a method for designing a compressor thereof.
[Nonpatent Document 1] S. V. Chernikov et al., “Soliton pulse compression in dispersion-decreasing fiber” Opt. Lett., Vol. 18, No. 7, pp. 476-478 (1993).
[Nonpatent Document 2] S. V. Chernikov et al., “Comblike dispersion-profiled fiber for soliton pulse train generation” Opt. Lett., Vol. 19, No. 8, pp. 539-541 (1994).
[Nonpatent Document 3] T. Inoue et al., “Optical Pulse Compression Based on Stationary Resealed Pulse Propagation in a Comblike Profiled Fiber” J. Lightwave Technol., Vol. 24, No. 7, pp. 2510-2522 (2006).